Is Sepsis-Induced Apoptosis Associated with Macrophage Dysfunction?

Abstract

Apoptosis (A sub o) is a pathological process by which cells undergo a form of inducible nonnecrotic cellular suicide. In vitro studies suggest that changes in the rate of macrophage (M_phi) A_o may be associated with elevated proinflammatory cytokine secretory capacity, such as interleukin-1 beta (IL-1beta) (via IL-1 converting enzyme activation). Furthermore, it has been reported that M_phi are activated during early (0-4 hours) experimental septic insult to act as sources of proinflammatory cytokines, such as IL-1. However, with the progression of sepsis, these same cells become refractory to further stimulation (appearing dysfunctional). Nonetheless, it remains unknown if this acquired immunosuppression (dysfunction) is associated with an acceleration in macrophage A_o. To determine this, male C3H/HeN mice were subjected to sepsis (cecal ligation and puncture, CLP) or sham-CLP and 4 or 24 hours thereafter M_phi were isolated from the peritoneum (PM_p sub h_i) and liver (KM_phi). Macrophage monolayers were lysed either after stimulation with lipopolysaccharide (LPS) (10 microg/mL, 24 hours) in vitro or immediately (ex vivo) before LPS stimulation and the cytoplasmic cell fraction was retained. The extent of A_o was determined using a cell-death enzyme-linked immunosorbent assay, which detects the presence of cytoplasmic oligonucleosomes and changes in the propidium iodide staining intensity. The results indicate that, early after CLP (4 hours) only PM_phi stimulated with LPS in vitro showed evidence of increasing A_o. At 24 hours (late) after the onset of sepsis, the ex vivo extent of A_o in PM_phi was increased but it was decreased in KM_phi. However, the addition of LPS in vitro results in a marked increase in both septic PM_phi and KM_phi A_o. This latter result suggests that the inability of M_phi to release cytokines in response to stimulants, such as LPS during late sepsis (24 hours), may be because of induction of accelerated A_o in these M_phi populations.